This is a proposal concerned with problems of sudden cardiac death and lethal arrhythmias. The major portion of the research proposed represents a continuation of research projects currently supported under a research grant, "Sudden Cardiac Death and the Onset of Myocardial Infarction" (HL=18805), which was initiated June 30, 1975. Much of this research was originally started under a contract "Sudden Cardiac Death and the Onset of Myocardial Infarction" (N-01-hl-12474) in 1971. For a number of the projects, there will be continued surveillance and follow-up of patients enrolled in registries and the use of large valuable data bases which have been developed for the research projects. Various elements of the on-going aspects of the program have been called "Seattle Heart Watch", and this designation will be used to describe certain activities in the proposal. The projects are planned to provide new information concerning the characteristics of heart disease and prognosis of survivors of sudden cardiac death, mechanisms and circumstances related to sudden cardiac death, development and evaluation of methods for providing earlier recognition of individuals at risk for having sudden death, cardiac pathology in sudden cardiac death, distribution of myocardial blood flow as assessed by radionuclide techniques, the possible role of platelet thrombi to arrhythmias and sudden death, detection and therapy of arrhythmias in sudden death survivors, long-term prognostic significance of clinical findings, coronary anatomy and ventricular performance in medically and surgically treated patients with coronary artery disease and the health status of survivors of sudden cardiac death. Studies of repetitive ventricular response to electrical and mechanical stimulation will be initiated to evaluate electrical instability in sudden death survivors.